Fabrication method of LiCoO2 nano powder by surface modification of precursor

ABSTRACT

LiCoO 2  nano powder of a high-temperature polymorph having a small and uniform size of grains which is obtained by modifying a surface of a precursor by mixing inert soluble salt on a Li-Co acetate precursor, and by heating the surface-modified precursor is provided, and a battery manufactured by using the powder as the cathode material has very excellent charging/discharging characteristics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fabrication method of LiCoO₂ nanopowder by a surface modification of a precursor.

2. Description of the Background Art

A secondary battery which is widely used uses LiCoO₂ powder as thecathode materials. In fabricating the powder, a method of mixing lithiumcarbonate and cobalt trioxide and calcining the mixture at a hightemperature is used. Such method is performed at higher than 800° C. toobtain LiCoO₂ of high temperature polymorph. In high temperatureheating, growth of grain is occurred and in case of widely used powder,the size of the grain reaches 5˜10 μm. In addition, since the growth ofthe grain depends on contact state among grains, homogeneous grain sizedistribution of the grain which is finally obtained after calciningbecomes very ununiform. Such characteristic is one of reasons thatdifference of powder characteristic for each batch for powderfabrication is caused.

In case the size of the grain is large, since removing/inserting rate ofLi⁺ ion is relatively low in charging/discharging, it does not matter ata relatively low discharging rate, but capacity is faded at higherdischarging rate. Also, there are many defections by broken bonds on thesurface of the grain, and Li⁺ ions are trapped on the defective portion.Therefore, capacity fade is generated by charging/discharging.

In case the size and distribution of the grain are very uniform, thereis an advantage that reproducibility in subsequent battery fabricationprocesses become excellent. Also, in case the size of the grain becomessmaller, the moving rate of the Li⁺ ion is relatively increased, andaccordingly capacity fade according to high-speed discharging can beprevented, thus to enable manufacture of a high-output battery.

By the above reasons, there is an increasing demand for new LiCoO₂powder having a very small grain size and very uniform graindistribution.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide LiCoO₂powder of a high temperature polymorph and a fabrication method thereofhaving a very small and uniform grain size.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described herein,there is provided a fabrication method of LiCoO₂ nano powder, includingthe steps of preparing a Li—Co acetate precursor modifying a surface ofthe precursor by mixing the precursor with inert soluble salt obtainingLiCoO₂ powder heating the surface-modified precursor, and removing inertsoluble salt from the powder.

The inert soluble salt is selected among K₂SO₄, (NH₄)₂CO₃, NaCl and KCl,and the amount of Li in the precursor is 100%˜130%.

In the present invention, the LiCoO₂ powder is obtained by cooling anddrying by mixing initial materials, modifying the surface with anappropriate modifier, and heating the resultant material differentlyfrom the calcining method after simply mixing the conventional materialsof the high temperature polymorph.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1A is a view showing an XRD pattern of LiCoO₂ powder which isfabricated by the present invention;

FIG. 1B is a view showing an XRD pattern of conventional LiCoO₂ powder;

FIG. 2A is a SEM photo showing a shape of the LiCoO₂ powder fabricatedin accordance with the present invention;

FIG. 2B is a SEM photo showing a shape of the LiCoO₂ powder fabricatedwithout surface modification;

FIG. 3A is a graph illustrating charging/discharging characteristics ofthe LiCoO₂ powder fabricated in accordance with the present invention;and

FIG. 3B is a graph illustrating charging/discharging characteristics ofthe conventional LiCoO₂ powder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

Fabrication of LiCoO₂ Powder

Firstly, Li—Co acetate precursor was prepared by cooling and dryingprocesses as follows. A mixture solution of Li and Co was made by mixingLi acetate (W=102) and Co acetate (W=249) at a rate of 1.2:1 indistilled water (here, W designates molecular weight). For completelydissolving acetate, magnetic stirring was conducted at a temperature of50° C. for more than 30 minutes.

Then, the mixed solution was sprayed to the cooling dryer having liquidnitrogen. Through the spraying process, very small droplets were rapidlycooled under the liquid nitrogen condition and then a nano-sized solidmaterial was obtained. Then, the mixed solution which was educed waspoured into a flask which was cooled to 30° C. together with theremained liquid nitrogen. At this time, in case the remained liquidnitrogen is small, the amount of the initial liquid nitrogen must beappropriately adjusted since the educed mixed solution can be resolvedagain. After having the liquid nitrogen completely get out of the cooledflask, a vacuum condition was formed using a vacuum pump. At this time,the degree of vacuum was maintained as 6×10⁻² torr. In case thetemperature of the flask is 30° C., the condition is also completelydried condition.

The fabricated Li—Co acetate precursor was mixed with K₂SO₄, one ofinert soluble salt, as the surface-modifying material at a weight ratioof 1/6 of the precursor. Similar grain sizes were obtained in both handmixing and ball mixing. The hand mixing and ball mixing were performedrespectively for 20 minutes and 12 hours.

The Li—Co acetate precursor mixed with the modified material was heatedin the air to 400° C. at a temperature raising rate of 3° C. per minute,and then maintained for 6 hours. By the heating, carbon substance wasremoved and a low temperature polymorph of LiCoO₂ powder was formed. Toobtain high temperature polymorph of the fabricated powder, the powderwas heated in the air to 800° C. for 12 hours. After completing suchprocess, 3 times of washing was performed in distilled water to removeinert soluble salt on the surface of the powder (100 cc of distilledwater per 3 g of mixture), and then pure LiCoO₂ powder was obtained bycentrifugation.

Characteristic Analysis

To check crystal characteristic of the fabricated LiCoO₂ powder, anX-ray diffraction (XRD) pattern was investigated. FIG. 1A is a viewshowing an XRD pattern of the LiCoO₂ powder in accordance with thepresent invention, and FIG. 1B is a view showing the XRD pattern of theLiCoO₂ powder widely in use which is used to manufacture a secondarybattery. The crystal characteristic of the powder fabricated inaccordance with the present invention is identical as the typicalcrystal characteristic of the commercialized powder, and it is notedthat the XRD peak intensity ratio between the determined directions werealmost similar. In addition, a very strong (003) peak was found near2θ=18.6°, and this shows that the fabricated LiCoO₂ powder is formed ina lamellar structure having a hexagonal crystal structure and ishigh-temperature polymorph of the typical LiCoO₂ powder. In case of thelow-temperature LiCoO₂, the (003) peak is generated at 2θ=18.8°. Thismeans that the surface-modifying material covered by the Li—Co precursordoes not affect on the crystal characteristic of LiCoO₂.

The size and shape of the LiCoO₂ grain which was phase-transited to acomplete high-temperature polymorph was analyzed with a scanningelectron microscopy. FIG. 2A is a SEM photo showing the powder obtainedfrom the surface-modified Li—Co precursor which is covered with K₂SO₄,and FIG. 2B is a SEM photo showing the powder obtained by heating Li—Coprecursor without surface-modifying. In FIG. 2A, in case the precursoris not covered by the surface modifier, grains having a size as 4˜7 μmin average and maximum to 10 μm by heating were shown. In addition, anununiform grain shape of a very angular shape can be shown. On thecontrary, as shown in FIG. 2A, in case the precursor is covered by thesurface modifier, the size of the grain is 300 nm in average, and theshape of the grain is also a shape of a well developed facet.

Generally, difference of the grain shape is generated by difference ofcrystal characteristic in case of an identical material. However, incase of the two powders shown in FIGS. 2A and 2B, since an identicalheating process is performed as the identical precursor, it isdetermined that difference in the grain shape was caused by whether theprecursors were contacted each other in the growing process of thegrains rather than difference of crystallinity (almost identical peakpattern is shown in the XRD result). That is, in case of the Li—Cocovered by the K₂SO₄ which is a surface-modifying material, since thereis no contact among the precursors, the growth of the grains isrestrained, and accordingly only changes of crystallinity and shape ofthe grains were generated. On the other hand, in case the surfacemodifier is not well covered, it is determined that changes of the grainshape according to the growth by the mutual absorption among theprecursors as well as changes in the crystallinity and shape of thegrains by contact among the grains were also generated. An overalldistribution of grain size, that is, granularity as well as the size andshape of the grains also shows very different characteristics. As shownin FIG. 2B, in case of the powder without the surface modifier, it canbe shown that it is composed of grains of various sizes. On thecontrary, in FIG. 2A, it is noted that the powder is composed of grainsof a uniform size. That is, by the process used in the presentinvention, manufacture of LiCoO₂ having a homogeneous grain sizedistribution is possible.

Manufacture of a Battery

Such unit battery was manufactured using the fabricated powder. Thefabricated LiCoO₂ powder was mixed with a conductive agent AB(Acethylene Black) and binder PVdF (13% of polyvinylidene fluoride)respectively at rates of 87 wt %, 7 wt % and 6 wt %, and NMP(1-methyl-2-pyrrolinone) and acetone were added. Then, the resultantmaterial was mixed to have a uniform viscosity at a rate of 5000 rpm ina high-speed agitator. At this time, viscosity was maintained in about10000˜15000 cp as the apparent viscosity.

The mixture was coated on an aluminum thin plate which is a currentcollector by a doctor blade having a thickness of 200 μm. Then, afterdrying the plate at 80° C. for an hour, the plate is cut into a size of3×4 cm², and the parts are rolled at 100° C. Then, the cathode wascompleted by drying the parts in the dried oven for 24 hours again.

After completing fabrication of the cathode, a material formed byinfiltrating a solution having a rate of EC:EMC:DMC as 1:1:1, in which 1M LIPF₆ salt is dissolved was used as the electrolyte on the PP(polypropylene) separator of 20 μm. Manufacture of a battery wascompleted by sealing with a vacuum pack under the condition that acarbon anode is abutted on the electrolyte and the cathode and anodeterminals are exposed. Every processing was performed at humidity oflower than 0.3%.

Charging/Discharging Test

To test the electrical-chemical characteristic of the LiCoO₂ powder inaccordance with the present invention, charging/discharging tests of thebattery were performed, and the results were shown in FIG. 3A. Thedischarging rate (C rate) was varied to 0.5, 1, 1.5 and 2C to comparethe characteristic according to the discharging rate. (In the drawing,the discharging rate is shown respectively as 0.5C, 1C, 2C, 3C) Thecut-off voltage region is set as 4.2-3.0V. The results of thecharging/discharging tests of the battery which was made by using acathode material fabricated in the Semi corporation in Japan is shown inFIG. 3B for comparison. In the drawing, it is noted that the overalldischarging aspects are very similar. It is shown that the overallcapacities as well as discharging aspects are almost identical in 0.5and 1C. The discharging capacity of the battery which used the powderwhich was fabricated in the present invention as the cathode materialswas larger, and such difference was more apparent at the dischargingrate of 2C. That is, it is noted that the battery which used the powderwhich was fabricated in the present invention as the cathode materialshas a more excellent high-speed discharging characteristic.

In addition, since the shape of the grain is much more round than in thecase of the common powder, defection by broken boding of LiCoO₂ on thesurface of the grain can be reduced, it can be expected that capacitydrop after charging/discharging is smaller than in case the commonpowder is used.

The above described fabrication method can be applied to fabricateanother lithium-alloy oxide powders. For instance, Li-M which is alithium alloy used for the battery (here, M is Ni, Mn, or M-Ni—Mn alloy)may be fabricated as nano-sized powder using the method disclosed in thepresent invention.

In accordance with the present invention, LiCoO₂ powder of ahigh-temperature polymorph having a very small and uniform size ofgrains can be fabricated. A battery with an excellent electriccharacteristic can be manufactured by using such LiCoO₂ as the cathodematerials. In addition, the present invention can be applied tofabricate various metal oxide nano grains as well as cathode materialsof the battery.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the metes and bounds of theclaims, or equivalence of such metes and bounds are therefore intendedto be embraced by the appended claims.

What is claimed is:
 1. A method of preparing LiCoO₂ nano powder,comprising the steps of: preparing a Li—Co acetate precursor; modifyinga surface of the precursor by mixing the precursor with an inert solublesalt; obtaining LiCoO₂ powder by heating the surface-modified precursor;and removing the inert soluble salt from the powder.
 2. The method ofclaim 1, wherein the surface modification of the precursor is performedby mixing the precursor and inert soluble salt in a mechanical-chemicalmethod.
 3. The method of claim 1, wherein the inert soluble salt is onewhich is selected from K₂SO₄, (NH₄)₂CO₃, NaCl and KCl.
 4. The method ofclaim 1, wherein the Li—Co acetate precursor is prepared by (i)mixing Liacetate and Co acetate with water to form a solution, and (ii) coolingand drying the mixed solution.
 5. The method of claim 1, wherein theheating includes a first heating for obtaining powder from theprecursor, and a second heating for phase transformation to a hightemperature polymorph.